Q: What causes Bovine Tuberculosis or TB?
A: TB is caused by an infection with the bacteria called Mycobacterium bovis (M. bovis). Infected animals may not show any outward signs of disease until the most advanced stages.
Q: Can M. bovis survive in the environment?
A: M. bovis is very resilient. Survival is affected greatly by temperature and moisture. When exposed to summer temperatures, sunlight and drying conditions, the bacteria dies, but away from sunlight and in moist conditions in soil and manure, M. bovis can survive for many months, especially in the cold.
Q: How is the disease transmitted from wildlife?
A: In Michigan, as well as in areas of New Zealand, the U.K. and other locations, wild animals are believed to be a primary major source of M. bovis transmission to cattle. A wildlife reservoir of the disease occurs because of three factors: 1. The disease has established itself in a wildlife population, 2. The live bacteria can be shed by the animal, and 3. There is opportunity for M. bovis to be transmitted to cattle through both direct and indirect pathways.
An example of direct transmission of M. bovis would be when aerosols or droplets that are exhaled or coughed from an infected animal and contain the bacteria go directly from infected animal to a susceptible animal. This type of transmission would most likely occur in crowded conditions or high populations or in contained areas.
Indirect transmission could occur when bacteria from an infected animal is transmitted to a susceptible animal through some indirect means. Examples would be when an infected animal’s respiratory secretion or saliva containing M. bovis is deposited on a feedstuff which is then subsequently ingested by a susceptible animal, or through ingesting milk that contains the live M. bovis.
In Michigan, it is believed that indirect transmission of M. bovis is the most common cause of herd infections.
Q: Can M. bovis be spread through small wild animals like opossums and raccoons?
A: While the disease has been identified in these animals as well as in scavenging carnivores such as coyotes, bobcat, fox and bear, it is not believed that these animals can pass it on to cattle or that it has become endemic in their populations. Producers are encouraged to control small animals on their farms to suppress possible transmission of various diseases.
Q: How contagious is M. bovis within a cattle herd?
A: Diseases spread when conditions favor exposure and when animals are susceptible. In Michigan herds located in the geographic area known to have TB in wildlife, annual testing of cattle has found the infection often in early stages. In most of the infected herds, only one or two infected animals have been found. From that, it was believed that the disease was not very contagious. However, the experience with a few herds in which a high proportion of animals has been found infected causes us to question that conclusion. In fact, when conditions and management result in greater exposure, particularly of young animals, the disease can apparently be fairly contagious.